In 1987, the isolation and synthesis of combretastatin A-1 (1) and B-1 (2) from the South African willow tree Combretum caffrum (Combretaceae) was reported (Pettit et al, 1987). Both natural products were shown to be significant cancer cell growth inhibitors and antimitotic agents, providing an ED50 value of 0.99 μg/ml and 1.7 μg/ml respectively against the murine P388 lymphocytic leukemia in vitro system, and inhibiting microtubule assembly in vitro with IC50 values of 2 μM (1) and 3 μM (Pettit et al, 1987). A comparison of diphenol 1 to the monophenol counterpart combretastatin A-4 (3a), the most active anticancer member of the combretastatin family, revealed a very similar antinmitotic activity (IC50 2-3 μM, but much greater cytotoxicity (ED50˜0.0009 μg/ml, P388 cell line) (3a) (Pettit et al, 1989). However in early murine (P388 leukemia) in vivo experiments, combretastatin A-1 led to more consistent antineoplastic results (Pettit et al, 1987). The relevant structures are shown in Figure 1, below. 
Development of combretastatin A4 to the current Phase I human cancer clinical trials was accelerated following synthesis of the phosphate prodrug 3c from dibenzyl phosphate 3b and then uncovering its very promising cancer antiangiogenesis effects. The phosphate derivative was chosen due to the nature of its biolability and enhanced solubility characteristics. Once administered, the phosphate prodrug is presumed to be converted into the parent drug via non-specific phosphatases and then transported intracellularly. Phosphate 3c showed similar cytotoxicity when compared to the parent compound (GI50 0.0004 μg/ml, P388 cell line), while greatly increasing the aqueous solubility to 20 mg/ml. Prodrug 3c was also shown to induce vascular shutdown within murine metastatic tumors at doses less than one-tenth of the maximum tolerated dose.
The preclinical development of combretastatin A-1 (1) was hampered owing to instability (oxidation to the 1,2-quinone) (Sackett, 1993; Haines, 1988) of the 2,3-dihydroxy unit. This was supported by the fact that acetylation of 1 significantly enhanced cytotoxicity 10-fold, while reducing the inhibition of the tubulin assembly.
The synthesis of the combretastatin A-1 and B-1 phosphate prodrugs were undertaken in order to improve solubility for drug delivery and to increase stability. Thus, the present invention is directed to the syntheses of combretastatin A-1 prodrug (ED50<0.0100 μg/ml, P388 cell line, 4), combretastatin B-1 prodrug (ED50 0.335 μg/ml, P388 cell line, 5), as shown in Scheme 1, below, and a series of metal cation and ammonium cation salts of the diphosphoric acid precursor 4 to evaluate effects on in vitro human cancer cell growth and solubility behavior. 